Tracers are volatile compounds added to various substances for the purpose of tagging and tracking the course of that substance in the environment. Known tracer vapors are be detectable at very low levels--parts per trillion (pp 10.sup.12) or less. Such tracers, when used appropriately, generally have no impact on health or the environment and are economically practical in the tagging of substances such as air, gas, liquids, and even solids.
U.S. Pat. Nos. 3,991,680 and 4,256,038 relate to methods of detecting small bombs to provide security against terrorist activities which can cause the destruction of civil aircraft in flight or detonate explosives in places where large groups of people congregate. These methods involve the tagging of explosive materials such as blasting caps with a so-called "vapor taggant" which can be "sniffed" and detected by suitable equipment. The vapor taggant disclosed in U.S. Pat. No. 3,991,680 is sulfur hexafluoride (SF.sub.6) absorbed in a fluoro-polymer. The vapor taggant disclosed in U.S. Pat. No. 4,256,038 is a Perfluorocarbon Tracer ("PFT") which includes one or a plurality of the following compositions: perfluorocycloalkanes such as perfluorodimethylcyclobutane (PDCB), perfuoromethylcyclohexane (PMCH), and perfluorodimethylcyclohexane (PDCH); perfluoroaromatics such as hexafluorobenzene (HFB), octafluorotoluene (OFT), decafluorobiphenyl (DFBP), decafluoroxylene (DFX), octafluoronaphthalene (OFN), and pentafluoropyridene (PFP), perfluoroalkanes such as perfluorohexane (PFH), perfluoropentane (PFPT), and perfluorooctane (PFO), and perefluorocycloalkenes such as decafluorocyclohexene (DFCH) and octafluorocyclopentene (OFCP).
The disclosure of U.S. Pat. No. 4,256,038 is expressly incorporated herein by reference.
As disclosed in these patents, the detection system for explosives consists of:
(1) "taggants" (for example, the Perfluorocarbon Tracers, or "PFTs") that give off detectable inert tracers when applied to materials; and PA1 (2) a sensing system capable of detecting taggants in the atmosphere. PA1 (a) applying a taggant to the contraband or sensitive documents; and PA1 (b) subsequently using a light to detect the presence of said taggant by the spectral characteristics of absorption, transmittance, reflectance, emission or fluorescence of the taggant and therefore the contraband or sensitive documents.
Taggant use involves the detection of inert gaseous vapors in minor tracer quantities that are emitted over time. As there are a plurality of separate tracers in the PFT family, each with its own "fingerprint", the PFTs can be combined in a range of combinations and concentrations, yielding thousands of discrete "signatures". This allows discrimination between various taggants and enables the individual detection of multiple products, or the tracking of individually tagged products to provide exact identification and location.
The Perfluorocarbon Tracer technology is believed to be the most sensitive of all known tracer technologies because the ambient background levels of the routinely used PFTs are extremely low, for example in the range of parts per quadrillion--ppq range. Therefore, because of this low "noise", the sensitivity of detection is dependent on the sensing instrument technology. Many of the PFT compounds, which are invisible and environmentally and biologically safe to use, are presently commercially available.
Various methods of detection have been demonstrated conclusively in numerous application projects for PFTs including indoor heating and ventilation studies, underground leak detection and long range atmospheric studies.
The following provides a simplified description of how the PFT tracers have heretofore been detected and analyzed, in order to understand some advantages of PFTs over other gaseous and non-gaseous tracers. The PFTS can be analyzed by gas chromatography wherein the constituents of an air sample are thermally absorbed from a sample tube and are injected into the carrier gas stream via a sample valve (in a building structure, multiple sampling tubes are run throughout the different regions of the building being monitored). Before entering the chromatography column, all the components are present as a "slug". After passing through the column, the constituents are physically and temporally separated to an extent that depends on the nature and conditions of the column.
The high affinity of PFTs for reaction with electrons also makes them some of the most sensitive compounds for detection in an Electronic Capture Detector (ECD), which is a small (0.1 to 0.2) ml. reaction chamber containing an electron source. The cloud of electrons in the chamber is periodically collected, producing a current. When tracer molecules enter the cell, the reacted electrons cannot be collected. The resulting reduction in current is a measure of the PFT concentration.
However, the atmosphere contains many components, the concentrations of which exceed those of the PFTs, and that are detectable in the ECD used to measure the PFTs. Included are O.sub.2, nitrogen oxides, chlorofluorocarbons (CFCs), SF.sub.6, and others, each of which could interfere with the early eluting PFTs. Physical means are used (e.g., sampling onto an absorbent with subsequent purging) to remove most of the oxygen and some of the CFCs. A catalyst bed operating at about 200.degree. C. is needed to destroy many remaining interfering compounds so that the surviving PFTs can be detected.
Sample collection is accomplished by several means. Inexpensive passive Capillary Absorption (sampling) Tubes (CATS) allow the monitoring of surveillance areas in remote or congested locations. These sampling tubes are collected and sent to a laboratory for analysis. Alternatively, a real time Continuously Operating Perfluorocarbon Sniffer (COPS) can provide immediate indications of the source of PFT emissions. Both the COPS and the ECD are commercially available.
It is the physical and chemical inertness of the PFTs that not only prevents their loss in the atmosphere, but also helps in their separation and analysis from less stable interfering compounds and makes them biologically inactive; and thus safe to use. Their limited industrial use not only results in low ambient background concentration, but also precludes the possibility of numerous higher local concentrations that might confuse detection capability.
PFT technology has already been developed and utilized in various applications including: (1) detection of leaks in underground storage tanks; (2) detection of leaks in high-pressure, oil-filled electric transmission lines; (3) atmospheric tracing and air pollution dispersion studies; (4) building ventilation studies and (5) detection of tagged explosives blasting caps in airline luggage. Investigation is underway for exploring the application of PFTs to the detection of leaks in natural gas pipelines and to early warning fire detection systems.
Effective inspection of large containers and trucks for controlled substances and narcotics is essential for the success of drug interdiction efforts. A significant fraction of drugs are smuggled through this avenue. Without prior knowledge provided through intelligence activities, the chances for drug detection are very slim. A successful drug interdiction program therefore requires efficient, rapid and cost-effective inspection techniques for large objects. The technique heretofore used to thoroughly inspect containers is manual, highly labor intensive and can hardly be expanded to meet the challenge of abating the flow of illicit drugs from one country to another. Hence, the only way to achieve the goal of an effective counter-drug effort is to develop a rapid, automatic, non-intrusive inspection system to inspect shipments and cargo containers without removing all of the contents for manual inspection,
U.S. Pat. No. 5,409,839, expressly incorporated herein by reference, is entitled and describes "A Method for the Tagging and Detection of Drugs, Crops, Chemical Compounds and Currency with Perfluorocarbon Tracers." The methods of detection mentioned in that patent are gas chromatography and electron capture detection. The limitations of these methods of detection are set forth herein.